Recently, a brushless DC motor has been used for an electric tool in which a tip tool such as a drill or driver is rotationally driven by a motor to perform a desired operation. The brushless DC motor is a DC (Direct Current) motor which has no brush (brush for rectification). The brushless DC motor includes a coil on a stator side and a magnet on a rotor side. The brushless DC motor is so configured that a power driven by an inverter is sequentially energized to a predetermined coil to rotate the rotor. In the brushless DC motor, a switching element for turning on/off the energization of the coil wound around a stator is placed on a circuit board near the motor. The switching element is located on an approximately circular circuit board attached to a rear side (a side opposite to the tip tool) of the motor (for example, see, JP-A-2010-99823).
An electric tool using the brushless DC motor typically includes a star-connected three-phase coil and a total of six switching elements are used in such a way that one switching element is respectively provided on the positive side and the negative side of each phase. Since a relatively large current flows in the brushless DC motor, a heating value of the switching element increases. In order to effectively cool the switching element, an air inlet is provided to a portion of a housing near the inverter circuit board. The circuit board is arranged perpendicular to a rotating shaft of the motor and the switching element is placed on the circuit board. A fan is mounted on the rotating shaft of the brushless DC motor. Outside air is suctioned through the air inlet by the fan to cool the switching element or the brushless DC motor.
In a related-art electric tool, an inverter circuit board mounted with a switching element is disposed at rear to the motor. The circuit board is mounted with an inverter circuit or hall IC, etc., and is held on a protrusion of an insulator. Such structure will be described by referring to FIGS. 5 and 6.
FIG. 5 is a side view illustrating an inverter circuit board and a stator of a motor in a related-art electric tool. FIG. 6 is a rear view of FIG. 5. In these drawings, a rotor and a rotating shaft of the parts constituting the motor have been omitted. In the brushless type motor, an end of a stator coil 3c passes inside an insulator 15, passes thorough an opening formed in an inverter circuit board 4 and is fixed at an opposite side of the inverter circuit 4 by a solder 9. A total of six stator coils 3c are soldered to the inverter circuit board 4. In addition, six switching elements 5 are mounted on the inverter circuit board 4.